Helper-dependent phage P4 is a satellite virus depending on temperate phage P2 as a helper. P2 provides the proteins for satellite phage head and tail synthesis, as well as cell lysis. P4 causes P2 head proteins to form a small head which is not found during normal P2 head morphogenesis. Thus P4 causes specific head size determination. We have purified two P2 head proteins which are needed for P2 DNA maturation and packaging and will study their mode of action. P4 induces an RNA polymerase activity which may initiate DNA replication by synthesizing polyriboguanylic acid. P4 amber mutants as gene alpha do not synthesize DNA nor produce this enzyme. We have developed a P4 in vitro replication system in order to purify the alpha gene product and to determine its relation to DNA synthesis and to the RNA polymerase activity. P4 can cause P2 genes to be expressed under conditions where they would normally remain repressed. This effect, called transactivation, involves the interaction of phage proteins with E. coli RNA polymerase. We will identify and characterize these P4 protein(s) which regulate P2 gene expression.